While high temperature fuel cells are a very efficient means of generating electricity, it is difficult to find materials which can withstand the extreme conditions encountered in the fuel cell. In particular, tubes of a porous insulating material are required which can withstand temperatures of about 1000.degree. C. Under an older design, these tubes were open at both ends and were made from high temperature refractory materials such as stabilized zirconia. In making the tube, a paste was extruded to form a tube which was hung and fired to produce a ceramic structure. The bottom of these tubes usually curled upward due to stresses induced in the firing. It was impossible to use crooked tubes in the fuel cell because they could not be stacked together so as to achieve uniform electrical contact between them. This was not a problem, however, because the remainder of the tube was straight and the bottom of the tube was simply cut off and discarded.
However, in order to make the fuel cells more efficient, the design of the tubes was changed so that one end of the tube had to be plugged. It was possible to place a plug of unfired paste in one end of a fired tube and refire the tube to fuse the plug to the tube, but this was an expensive procedure. It was not possible to place the plug at the top of the tube because a collar had to be placed around the top of the tube to hang it during firing, and the collar could not be removed without damaging the tube. Of course, if the plug was placed at the bottom of a tube, the tube would still curl and the bottom portion containing the plug would have to be cut off.